Selective oxidative conversion of triaryldihydro[C59N] fullerenes: A model case for oxygenation of carbon allotropes

Article abstract of DOI:10.1039/c3cc48461k

The photooxidation of triaryldihydro[C₅₉N]fullerenes was achieved by treatment with air and light leading to a new selective core functionalization of azafullerenes, which serves as a model case for oxygenation of carbon allotropes.

Full text of DOI:10.1039/c3cc48461k

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Selective oxidative conversion of
triaryldihydro[C59N]fullerenes: a model
case for oxygenation of carbon allotropes†
Regina Eigler,a Frank W. Heinemannb and Andreas Hirsch*a
Cite this: Chem. Commun., 2014,
50, 2021
Received 5th November 2013,
Accepted 19th December 2013
DOI: 10.1039/c3cc48461k
www.rsc.org/chemcomm
The photooxidation of triaryldihydro[C59N]fullerenes was achieved
The azafullerene dimer can be obtained by a three-step
1,2
by treatment with air and light leading to a new selective core reaction starting from C60
.
Covalent functionalization of
functionalization of azafullerenes, which serves as a model case for
oxygenation of carbon allotropes.
C59N monoadducts such as C59NAr has been investigated quite
extensively;13,14 however, heterofullerene derivatives with a higher
degree of addition have only been generated in a few cases,15–21 in
To date, azafullerenes, in which one carbon atom of the C60 skeleton contrast to the corresponding rich chemistry of the isocyclic fullerene
is substituted by nitrogen, are the only class of heterofullerenes that C60. However, the establishment of a Cs-symmetrical 6,8,12,15,18-
are accessible in macroscopic quantities.1,2 In general, carbon allo- addition motif, also known for C60 fullerene derivatives,22,23 was first
tropes like fullerenes, carbon nanotubes and graphene represent an demonstrated by our group with the successful synthesis of tetra-
emerging research field.3–5 Particularly, graphene based materials chlorinated aryladduct C59NArCl4 bearing an integral pyrrole moiety
have gained enormous attention due to their fascinating electronic within the p-system of the fullerene surface.16 Recently, we explored
properties.6 In this regard, graphene oxide7 plays an important role as the formation of pentaarylazafullerene C59NAr5 by an acid catalyzed
a promising precursor for wet chemical synthesis of graphene with reaction of a C59N precursor with electron-rich aromatic compounds.
minimal defects.8 Its structure contains epoxy and hydroxyl groups in During the course of these reactions, triaryldihydroazafullerenes
addition to carboxylic acids at the edges of the graphene oxide layers C59NAr3H2 1–4 were formed as stable intermediates.20,21 Furthermore,
according to the Lerf–Klinowski model.9,10 However, the oxygenation they exhibit the same pentakisaddition pattern, but contain an
mechanism behind structures like graphene oxide is still not under- unusual pyrrole substructure with allylic hydrogen atoms. The reac-
stood. Even the structure elucidation is still in progress.11 In addition, tivity of these triaryldihydroazafullerenes is still unexplored, despite
the chemical doping of graphene with nitrogen leads to promising the possibility of substituting the H-atoms by aryl moieties as we have
materials that have already shown superior preferences towards recently demonstrated.20,21 We have now discovered that these
various applications.12 However, chemical reactions on N-doped triaryldihydro derivatives 1–4 exhibit a pronounced reactivity
graphenes are an unaddressed issue. In this context, fullerenes, in towards air and light induced oxygenation. As a consequence, for
general, provide a very suitable model system to study oxygenation the first time a new class of compounds, namely oxofunctionalized
reactions and the stability of products due to their monodispersity azafullerenes were formed, isolated and characterized. In an initial
and the possibility of using unambiguous characterization methods photooxidation experiment of a mixture of the isomers 1–4
such as mass spectrometry, NMR spectroscopy and X-ray single (Scheme 1), we recognized that isomer 2 was converted much
crystal analysis. Here, we report on selective oxygenation reactions faster than 1, 3 and 4 (Fig. S1, ESI†). After a short exposure (15 min)
of azafullerene derivatives providing valuable insights into carbon of the reaction mixture to light and air, isomer 2 was oxidized to its
allotrope oxides and their formation mechanism.
green oxygenated product 5. As a consequence, we conclude that
this isomer 2 with both hydrogen atoms in the a0 position
(Scheme 1) is the most reactive of all four isomers.
a Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular
Earlier studies have already shown that the tetraarylmono-
hydro-[C59N]fullerene C59NAr4H with only one hydrogen atom
in the a0 position is more reactive towards further arylation than its
isomer bearing the hydrogen in the b0 position.20,21 The reaction
product 5 exhibits a MALDI-MS peak consistent with one additional
oxygen atom and one hydroxyl group. Advantageously, the isolation of
the main product 5 was achieved by column chromatography using
¨
¨
Materials (ICMM), Friedrich-Alexander-Universitat Erlangen-Nurnberg (FAU),
Henkestraße 42, 91054 Erlangen, Germany. E-mail: andreas.hirsch@fau.de;
Fax: +49 (0)9131 8526864; Tel: +49 (0)9131 8522537
b
¨
Department of Chemistry and Pharmacy, Friedrich-Alexander-Universitat
¨
Erlangen-Nurnberg (FAU), Egerlandstraße 1, 91058 Erlangen, Germany
† Electronic supplementary information (ESI) available. CCDC 967868. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3cc48461k
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Scheme 1 (a) Reaction scheme of the photooxidation of the mixture of
triaryldihydroazafullerene isomers 1–4 leading to main product 5 and (b)
the depicted 1H NMR spectrum of 5 shows a 4J coupling constant of 1.3 Hz
between the hydrogen addend and the hydroxyl group.
silica gel, and no HPLC separation was necessary as it is required for
the separation of the pure isomers 1–4. Oxidation side-products from
the other isomers 1, 3 and 4 were obtained in minor amounts, too.
To gain more insight into this reaction we took a closer look at
isomer 2. Single crystals of compound 2 were obtained from a mixture
of isomers 2 and 3 in CDCl3. The X-ray structure of 2 is shown in
Fig. 1a with the pyrrole unit in the middle, highlighted in green, and
the two hydrogen addends in the a0 position. Interestingly, isomer 2
crystallizes in a straight-stacking motif with the stacking axis in
direction b of the unit cell (Fig. 1c). The closest intermolecular
Fig. 1 (a) Single crystal structure of isomer 2 with two H- atoms in the a0
position, stick representation (C yellow/green, N blue, O red, H white); (b)
DFT calculated HOMO (B3LYP/6-31-G(d)) of 2; (c) space-filling represen-
tation of the packing motif (C yellow/purple, N light blue, O red, H white)
and (d) part of the crystal structure showing the closest intermolecular
Cꢀ ꢀ ꢀC distances. Solvent molecules (CDCl3) are omitted for clarity.
In preliminary experiments the oxygenated product 5 was
CꢀꢀꢀC distance is between two azafullerene cores of adjacent stacks further irradiated in order to test its stability under ambient
and measures 3.19 Å (Fig. 1d). The co-crystallized solvent molecules and inert (argon) conditions. The experiment shows that 5
(CDCl3, omitted for clarity in Fig. 1) are disordered and confined to completely decomposes within 1.5 h. In a control experiment
channels along the stacking axis.
a degassed sample of the mixture of 1–4 showed no reaction at
The pure isomer 2 can be converted to 5 like the mixture of 1–4 all upon irradiation for over 30 min. Therefore, we conclude
described above. The transformation of isomer 2 to the oxidation that oxygen is crucial for this initial reaction.
product 5 was monitored by 1H NMR experiments (Fig. S2, ESI†).
The first step of the observed oxygenation of 2 can be explained by
Full characterization of the isolated oxygenated product 5 was a Schenck ene type reaction.24 It is well known that azafullerenes as
provided by 1H, 13C, COSY, HSQC, HMBC and ROESY NMR well as fullerenes can sensitize very efficiently the formation of singlet
analyses, as well as by MS, HRMS, UV/Vis and IR spectroscopy. oxygen upon illumination.25,26 Moreover, the highest occupied mole-
The experimental data are consistent with the structure dis- cular orbital (HOMO) of 2 is located at the pyrrolic double bonds
played in Scheme 1 involving a newly formed double bond and according to theoretical DFT calculations at the B3LYP/6-31G(d) level
one additional epoxy and hydroxyl group on the azafullerene (Fig. 1b). The generated singlet oxygen can attack the allylic and the
core. The hydroxyl group is adjacent to the hydrogen on the rather electron poor double bond of 2 to give a hydroperoxide
fullerene. Coupling between the two protons Ha and Hb with 4J = intermediate 6 (Scheme 2). In contrast, electron-rich olefins under
1.3 Hz is shown in the 1H NMR spectrum (Scheme 1). We assume these conditions form 1,2-dioxetanes.27–29 The latter mechanism is
that the proton of the hydroxyl group is involved in a hydrogen also observed for the formation of C60-N-MEM-oxo-lactam, the pre-
bond presumably oriented towards the epoxy group because it cursor of the C59N dimer.30 Furthermore, we suggest that the lone pair
gives rise to a very sharp doublet. The 13C NMR spectrum clearly of the nitrogen atom plays an activating role to give the iminium ion
reveals C1 symmetry and the resonance signals of the epoxy intermediate 7 after hydroxide elimination. Subsequently, the nucleo-
structure and the hemi-aminal structure appear at d = 75.22, philic hydroxide can attack the imminium intermediate 7 to form the
77.51 and 91.95 ppm. The signals of the pyrrole-like C-atoms of oxygenated product 5. Scheme 2 shows a plausible ionic mechanism,
the starting material disappeared and the signals of the newly however, a radical mechanism cannot be excluded. The oxidation of
formed enamine double bond could be clearly observed at d = pyrrole derivatives is well studied and various oxidation products
109.34 ppm and 160.19 ppm. No indication of carbonyl groups depending on the substitution pattern of pyrroles have been
was found by IR and 13C NMR spectroscopy.
described.28,31 However, an oxidation product such as 5 bearing
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Due to the preactivated structure of the pyrrole subunit, triaryldi-
hydroazafullerenes are ideal model compounds to analyze defined
oxygenated products and the mechanism behind this photo-
oxidation process. Investigations on oxygenation reactions and
their products are of great interest especially for carbon allotropes
regarding the use of graphene oxide.
Scheme 2 Proposed reaction mechanism for the formation of oxygenated
product 5.
The authors thank the Deutsche Forschungsgemeinschaft
(DFG – SFB 953, Project A1, ‘‘Synthetic Carbon Allotropes’’) and
the project ‘‘Solar Technologies Go Hybrid’’ (State of Bavaria)
for financial support.
Notes and references
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